The present invention relates to a method for controlling fuel injection for an automotive engine, and more particularly for a fuel injection system employed with an intake-air pressure detecting method. In an ordinary fuel injection system employed with an intake-air quantity detecting method, an airflow meter is provided in an intake passage at a position upstream of a throttle valve to detect the quantity of intake air (Q). Basic fuel injection time of period Tp is determined by a calculation of Tp =Q/N (N is engine speed).
In the fuel injection control system dependent on intake-air pressure (Japanese Patent Laid Open No.58-107825), the quantity of intake-air is indirectly detected by the value of the pressure of intake-air. The intake-air pressure increases with decrease of engine speed and decreases with increase of engine speed. However, the variation of the intake-air pressure delays because of the volume of the intake system.
FIG. 5 shows variations of engine speed N, intake-air pressure P and engine torque T at idling of an engine.
If engine speed at idling varies as the curve N because of external disturbance, intake-air pressure should vary as a curve P' theoretically. However, the intake-air pressure varies actually as the curve P with a delay because of the volume of the air intake system. Since the basic fuel injection time Tp is determined by the pressure P, the engine torque varies as the curve T with a phase delay of about 270.degree. with respect to the variation of the engine speed N. Namely, at a point A where the engine speed is at a maximum deceleration, the engine torque is minimum, and at a point B where the engine speed is at a maximum acceleration, the torque is maximum.
Accordingly, the variation of the idling engine speed is maintained without damping.